Troops are being deployed to guard Buckingham Palace, Downing Street, and Parliament after Prime Minister Theresa May warned that another terror attack is “expected imminently.”

Meanwhile, a knifeman was arrested on The Mall outside Buckingham Palace on Wednesday morning just moments before the Queen passed by in a car on her way to an engagement at St. Paul’s Cathedral.

A picture shows the man sitting handcuffed on the ground surrounded by officers, with one holding what is believed to be a large blade that had been seized. The incident is not believed to be related to terrorism.

The government says up to 984 armed military personnel will be deployed on Britain’s streets as part of ‘Operation Temperer’, which has been launched in the wake of the terrorist attack at a pop concert in Manchester. Twenty-two people were killed, including children, and 59 were injured when Salman Adebi, 22, blew himself up at Manchester Arena on Monday night.

Foreign Secretary Amber Rudd told Sky News that “people are likely to see some military on the streets.” The deployment is intended to free up armed officers for counter-terrorism operations.

On Tuesday night, the British government raised its terrorist threat level from ‘severe’ to ‘critical’ – the highest on the scale – meaning another attack is considered imminent. It was last raised to this level after the 2007 Glasgow airport bombing.

Rudd has noted that Monday’s attack was “more sophisticated” than previous ones, and “it seems likely – possible – that he wasn’t doing this on his own.”

On Wednesday morning, three more men were arrested in connection with the suicide bombing. Abedi’s brother, 23, was arrested on Tuesday. All four men are currently in police custody.

Mark Rowley, the head of national counter-terrorism policing, said in a statement that the investigation is “large scale, fast moving and making good progress.”

“I have asked for support from the military to be deployed alongside the police … This is part of an agreed and well-rehearsed plan, and military personnel will remain under the command and control of the police service,” he added.

Westminster Palace has been closed to the public in light of police advice, and will not re-open until further notice, a statement on its website said.

In addition, the Changing of the Guard ceremony at Buckingham Palace was canceled on Wednesday in order to redeploy officers, the Ministry of Defence said.

London’s mayor Sadiq Khan says he wants to reassure all Londoners and visitors that “everything possible” is being done to protect the city, and has urged people to remain “calm and vigilant.”

“Emergency services prepare day in, day out for these situations. Our plans are well rehearsed and well prepared,” he said.

“There will be additional police officers on London’s streets over the coming days including additional armed officers. You will also see some military personnel around London.”

The national threat level has been raised to 'critical'. Watch my statement on what this means for Londoners today. pic.twitter.com/pOqUvbqThz